U.S. Pat. No. 3,681,102 disclosed the production of transparent glass-ceramic articles wherein the zinc spinel gahnite (ZnO.Al.sub.2 O.sub.3) constituted the predominant crystal phase. That patent described the preparation of such articles as involving three general steps: first, a glass forming batch consisting essentially, expressed in terms of weight percent on the oxide basis, of about 2-20% ZnO, 8-25% Al.sub.2 O.sub.3, 55-75% SiO.sub.2, and 2-12% ZrO.sub.2 as the nucleating agent was melted at 1650.degree.-1800.degree. C; second, that melt was cooled to a temperature below the transformation range thereof and simultaneously a glass body of a desired configuration was shaped therefrom; and, third, that glass body was exposed to a temperature between about 975.degree.-1250.degree. C. for a sufficient length of time to cause the development of nuclei and the subsequent growth of gahnite crystals on those nuclei. (The transformation range has been defined as the temperature at which a liquid melt is deemed to have become a vitreous solid, that temperature generally being considered as residing in the vicinity of the annealing point of the glass.)
The crystals were described as having diameters less than 1000.ANG. and as comprising between 20-50% by weight of the articles; the residual glassy matrix having a highly siliceous content. The final products were stated to exhibit linear coefficients of thermal expansion (25.degree.-300.degree. C.) ranging between about 25-40.times.10.sup.-7 /.degree. C., and to maintain their transparency for extended periods of exposure at temperatures up to 1000.degree. C. and for brief intervals at 1200.degree. C.
The patent also disclosed the incorporation of 0.01-1.0% Cr.sub.2 O.sub.3 into the base compositions to produce Cr.sub.2 O.sub.3 -doped gahnite crystals, thereby imparting red photoluminescence to the glass-ceramics. The inclusion of Cr.sub.2 O.sub.3 did not adversely affect the transparency or other physical properties of the products.
As optional additional ingredients, the patent observed the use of up to 5% MgO, up to 10% total of BaO, CaO, SrO, K.sub.2 O, Rb.sub.2 O, and Cs.sub.2 O, and up to 5% total of Li.sub.2 O, Na.sub.2 O, B.sub.2 O.sub.3, and TiO.sub.2. The working examples, however, contained only MgO as an optional addition.